Normal shoulder kinematics allows for translational motion of the humeral head within the glenoid particularly at the end ranges of passive motion. These translations can be several millimeters and are thought to be due to capsular tightening on the opposite side of the direction of humeral head translation. Small amounts of translational motion also occur with active range of motion and are correlated with the degree of mismatch in the radii of curvature between the humeral head and glenoid. The native articular surface of the humeral head is not spherical in shape. Nevertheless, it is believed the central portion of the native humeral head is spherical and its curvature then decreases out toward the periphery. In fact, one study shows that the native humeral head of adults has a shape of a semi-ellipsoid with its anterior to posterior radius of curvature being approximately 2-3 mm less than its superior to inferior radius of curvature. See J. Iannotti, J. Gabriel, S. Schneck, B. Evans, and S. Misra, The Normal Glenohumeral Relationships, The Journal of Bone and Joint Surgery, Vol. 74-A, No. 4 (April 1992). The effect of a semi-ellipsoid native humeral head shape on the kinematics of the shoulder joint has not been well defined. Based upon the kinematics of the knee and femoral condyle anatomy, it would be suspected that asymmetric radii of curvature would allow for roll or translational motion of one surface on the other. It has been my long standing hypothesis that a semi-ellipsoid humeral prosthetic head design may allow for humeral head translation on the glenoid. This concept becomes important in prosthetic arthroplasty because translational motion of the humeral head and mismatch of the radii of curvature play an important role in wear and loosening of the glenoid component as well as the materials that can be used for a load bearing surface.
In conventional shoulder prosthetic designs, the humeral prosthetic head is spherical in shape and normal translational motion is allowed by a larger radius of curvature of the glenoid component. Although this will allow for translation before rim loading thereby decreasing the risk of loosening, it also increases the stress per unit area increasing the wear potential. In addition mismatched radii of curvature prevent the use of metal on metal or ceramic bearing surfaces and limits the materials to metal on plastic. One concept to manage or address the issue for allowing for humeral head translation versus decreased surface contact area is the biconcave glenoid design with a spherical head in the Bigliani Flatow shoulder. This design proposed to improve wear characteristics when there is perfect conformity with the arm through a mid range of motion, when the radius of curvature of the center of the glenoid was equal to that of the spherical humeral head, yet allowing for translation at the end ranges of motion when the radius of curvature of the glenoid increased. With use of the shoulder it has been suspected that the subtle differences in the radii of curvature of the poly glenoid would be lost due to plastic deformation of the part resulting in a uniform radius.
A requirement of humeral prosthetic surgery and design is the need to precisely place the prosthetic within the area defined by the humeral osteotomy and to reproduce the center of rotation of the normal native humeral head of a patient. When using a resurfacing component proper placement can be achieved by the surgical technique when choosing the site for preparing the humeral head. For a stemmed arthroplasty, this is achieved by surgical technique as well as prosthetic design which may require an eccentric taper. An eccentric taper on a spherical head can be effective in placement of the humeral prosthetic head in the optimal position within the plane of the humeral osteotomy by rotation of the humeral prosthetic head.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.